There have been conventionally known distribution optical fiber sensors for measuring a distortion distribution by measuring a frequency shift amount of Brillouin scattering created in an optical fiber along an optical fiber, and those for measuring a temperature distribution by measuring a light intensity ratio of Stokes lines to anti-Stokes lines of Raman scattering created in an optical fiber. For example, they are disclosed, for example, on pp. 325 to 327 of the “Collection of Next-Generation Optical Technologies” (published by Optoronics). The spatial resolutions of these distribution optical fiber sensors have had a limit of an order of several meters due to their measuring methods.
Accordingly, an inventor of the present invention proposed a distribution optical fiber sensor system having a high spatial resolution of an order of subcentimeters in Japanese Patent Application No. H11-150618 as one of the inventors, and this application was published in Japanese Unexamined Patent Publication NO. 2000-074697.
FIG. 23 is a construction diagram of the distribution optical fiber sensor system disclosed in Japanese Unexamined Patent Publication NO. 2000-074697.
In FIG. 23, this distribution optical fiber sensor system 1000 is provided with an optical fiber 1002, a pump light source 1003, a probe light source 1004, a light intensity detector 1005, an optical coupler 1006, an optical filter 1007, a calculating means 1008, and a control means 1020.
The optical fiber 1002 as a part of a sensor for detecting a distortion and a temperature is placed on a structure 1001 as an object to be measured. A discontinuous pump light emitted from the pump light source 1003 is incident on one end of this optical fiber 1002 via the optical coupler 1006, whereas a discontinuous probe light emitted from the probe light source 1004 is incident on the other end of the optical fiber 1002. The discontinuous pump light causes various scatterings such as Brillouin scattering, Raman scattering, and Rayleigh scattering by, for example, the nonlinearity of the optical fiber 1002. The caused various scatterings amplify the discontinuous probe light if the frequencies thereof coincide with that of the discontinuous probe light, and the amplified discontinuous probe light is introduced to the optical filter 1007 by the optical coupler 1006. The optical filter 1007 mainly transmits the Brillouin-amplified discontinuous probe light (Brillouin-scattered light) from these various scattered lights. The transmitted Brillouin-scattered light has its light intensity detected by the light intensity detector 1005, and a detection result is outputted to the calculating means 1008. The control means 1020 sets the frequency of the discontinuous probe light, controls the emissions of the probe light source 1004 and the pump light source 1003 so that the discontinuous probe light and the discontinuous pump light overlap at a measuring position on the structure 1001, controls a sampling timing in the light intensity detector 1005 so that the Brillouin-scattered light created as a result of interaction can be detected, etc.
The calculating means 1008 calculates the distortion and the temperature of the optical fiber 1002 based on the detection result of the light intensity detector 1005. In this calculation, the distribution optical fiber sensor system 1000 has achieved a high spatial resolution by dividing an overlapping section where the discontinuous probe light and the discontinuous pump light overlap into a plurality of small sections.
Since an acoustic phonon as a cause of Brillouin scattering is a mechanical propagation, it cannot momentarily start vibration and a transient phenomenon is known to exist (J. Smith, A. Brown, M. DeMerchant, X. Bao, “Pulse width dependence of the Brillouin loss spectrum”, Optical Communication Vol. 168 (1999), pp. 393–398). Thus, in order to more precisely measure a distortion and a temperature taking advantage of Brillouin scattering, this transient phenomenon needs to be considered.
In view of the above problems residing in the prior art, an object of the present invention is to provide a distribution optical fiber sensor system having a high spatial resolution and taking a transient phenomenon into account by using a first and a second pump lights having different frequencies.